1. Field of the Invention
The invention is generally directed to plasma etching of indium and tin oxides. The invention is more specifically directed to reactive ion etch (RIE) of indium tin oxide (ITO) by reactive gases that include hydrogen and a halogen (such as HCl gas).
2. Description of the Related Art
Opto-electronic devices such as liquid crystal displays (LCD's), charge coupled sensor devices (CCD's) and the like often include thin-film transparent electrodes disposed over a light transmitting or light receiving element.
The transparent electrodes are typically composed of an oxide of indium (InO) or an oxide of tin (SnO) or a mixture of these oxides or a compound having the general formulation: In.sub.x Sn.sub.y O.sub.z, where the z factor is greater than zero but less than 100%. The formulation, In.sub.x Sn.sub.y O.sub.z is commonly known in the art as ITO.
During manufacture, a thin-film of the material making up the transparent electrodes is deposited on a substrate. The thin-film is thereafter selectively etched so as to remove pre-specified portions and thereby define a desired wiring pattern.
It is generally desirable in mass-production situations to etch the transparent-electrode thin-film in such a way that the etching does not significantly damage any underlying structures.
It is also generally desirable to perform the etch as quickly as possible and with as few steps as necessary in order to reduce mass-production complexity and costs.
Until recently, the most common method for selective etch of ITO was to wet etch through a photo-lithographically patterned mask using chemically-reactive aqueous agents such as ferric chloride (FeCl.sub.3).
Wet etching has drawbacks though. It tends to leave a liquid residue, which residue often needs to be removed prior to further processing. The residue removal step complicates the overall process and disadvantageously increases costs.
Another drawback of wet etching is that its material removal rate tends to be highly sensitive to temperature variations. Tight temperature control is needed to compensate and prevent over or under etching. This also complicates the overall process and increases costs. (`Under etching` refers here to the condition where the transparent-electrode thin-film is not etched through thoroughly and undesired shorts appear in the resultant conductor pattern. `Over etching` refers here to the condition where the transparent-electrode thin-film is etched through thoroughly and undesired etching of the underlying substrate begins and/or time and resources are wasted in trying to etch to a depth beyond that needed.)
Yet a further drawback of wet etching is that it is isotropic. Over etching may lead to undesired undercutting beneath the etch mask. The undercutting may be so extensive that it leads to unintended open circuits in the conductor pattern.
More recently, attempts have been made to overcome the problems of wet etching by instead dry etching the material layer of the thin-film transparent electrodes with anisotropic reactive plasma.
U.S. Pat. No. 5,094,978, issued Mar. 10, 1992 to Miyagaki et al., METHOD OF PATTERNING A TRANSPARENT CONDUCTOR, reports successful dry etching of ITO with nitrogen trifluoride (NF.sub.3). Miyagaki indicates that it is not possible to dry etch materials such as ITO, In.sub.2 O.sub.3 and SnO.sub.2 with a reactive halogen gas (col. 3, lines 22-27).
A more recent U.S. Pat. No. 5,318,664, issued Jun. 7, 1994 to Saia et al., PATTERNING OF INDIUM-TIN OXIDE VIA SELECTIVE REACTIVE ION ETCHING, indicates limited success with plasma etching of ITO using HCl. Saia reports an etch rate of as much as 60 Angstroms per minute (60 .ANG./min) for plasma etching of ITO with HCl. Saia cautions, however, that HCl should not be used for concluding the etch-through of ITO because HCl is not selective with respect to any underlying silicon. Saia teaches to instead use a non-halogenated gas such as an acetone/oxygen plasma for completion of the etching through of the ITO layer.
Multi-step plasma etch processes such as that taught by Saia (5,318,664) are disadvantageous in that the switching over from one etch process (e.g., HCl plasma) to another (e.g., acetone/oxygen plasma) complicates the overall manufacturing process, increases costs and tends to reduce yields. The 60 .ANG./min etch rate reported by Saia for etching ITO with an HCl plasma is relatively slow for mass production needs.